Here is my situation, I've played for 4 years, I'm very dedicated (I play everyday and a lot on weekends), I believe I'm an advanced beginner, I can cover most pop songs and few very simple solos with a bit of practice, but I don't really understand the theory behind what I'm playing. I really feel I should learn some theory if I want to progress further. I can cover songs because I can read the tab and I practice it, but I want to learn how chords are constructed, why is everybody talking about major scales and pentatonic scales (I looked them up, but I don't get why I need to learn those). When I asked why we practice pentatonic scales, the answer I received was that it is a "pool of notes" rock guitar players used in their songs. Trust me, for a beginner like me it doesn't mean much.
I want a good book that can help me progress, I guess this forum is the best place to ask all of you that went through this what is the best book/tool to help me understand theory. Kingpatzer gave a few recommendations on books, is there any specific one that is better and simple enough for me.